The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device in which an illumination light source and a touch panel are stacked on a reflection type of liquid crystal panel which modulates light incident from its display-surface side and allows the modulated light to exit toward the display-surface side.
In recent years, so-called liquid crystal display devices using liquid crystal panels have been widely used as display devices which serve as monitors for image reproducing apparatus or various information terminals.
In general, such a liquid crystal display device uses a passive matrix type of liquid crystal panel which is known as an STN type, or an active matrix type of liquid crystal panel using non-linear devices such as TFTS.
These types of liquid crystal panel are not of the self light emitting type, and so they require an illumination light source to form a visible image formed on the liquid crystal panel. Liquid crystal panels are divided into a transmission type and a reflection type. In monitors for information terminals, the transmission type of liquid crystal panel is widely used for the purpose of high-luminance and high-contrast display, and for this purpose a backlighting illumination light source (hereinafter referred to as a backlight) is installed on the back of the liquid crystal panel, and a visible image is formed by modulating light from this backlight to form an image on the liquid crystal panel.
FIG. 10 is a cross-sectional view illustrating an example of a conventional liquid crystal display device provided with a transmission type of liquid crystal panel and a backlighting illumination device, i.e., a so-called backlight. The conventional liquid crystal display device has a backlight stacked on the back of the transmission type of liquid crystal panel, and is constructed to modulate, with an image formed on the liquid crystal panel, illumination light which is emitted from the backlight and which passes through the liquid crystal panel. The modulated illumination light is caused to exit toward the obverse side of the liquid crystal panel, thereby making the image visible.
On the other hand, a so-called touch panel may be stacked on the display-surface side of such a liquid crystal display device so that information can be externally inputted from the display surface of the liquid crystal panel.
Specifically, this kind of liquid crystal display device is constructed in the following manner, as seen in FIG. 10. In a liquid crystal panel 1, a liquid crystal layer is clamped between two transparent substrates 1A and 1B, and polarizers 1C are respectively provided on the obverse-surface side and the back-surface side of the liquid crystal panel 1. An illumination light source 3 is provided, which includes an approximately rectangular transparent light guide plate 2, a light source (a light source made of a cold cathode fluorescent tube, a light emitting diode or the like) 3A installed along one edge of the light guide plate 2, and a reflection sheet 3B, and this illumination light source 3 is provided on the back surface of the liquid crystal panel 1. Light from the illumination light source 3 is deflected toward the liquid crystal panel 1 at an intermediate position, while being propagated in the light guide plate 2, and illuminates the liquid crystal panel 1 from the back surface thereof. A light scattering area 10 formed by dot printing or the like is provided on the back surface of the light guide plate 2.
Furthermore, a reflector 11 for totally reflecting light which has exited from the light guide plate 2 on the back-surface side thereof and for returning the light to the liquid crystal panel 1 is installed on the back side of the light guide plate 2.
Such a backlight is stacked on the liquid crystal panel 1 with a light quantity distribution correction member such as a scattering film 12 or a prism plate (not shown), being interposed therebetween, whereby a transmission type of liquid crystal display device is constructed.
The above-described liquid crystal panel is undesirably limited in reduction in power consumption because the two transparent substrates 1A and 1B arc used and the backlight installed on the back surface of the lower transparent substrate 1B is normally on.
In addition, a semi-transmission type liquid crystal display device is known in which a lower substrate of a liquid crystal panel is formed as a semi-transmissive substrate so that light (external light) incident from the display side of the liquid crystal panel is normally reflected by the lower substrate and is made to exit toward a display surface; whereas, if the quantity of external light is insufficient, an illumination device installed on the back surface of the liquid crystal panel is turned on. However, this type of device has the disadvantage that an insufficient contrast is obtained.
On the other hand, in a reflection type of liquid crystal display device in which a lower substrate of a liquid crystal panel is formed as a reflector, or when a reflector is installed on the back side of the lower substrate, since external light incident from a display surface is approximately 100% reflected and used for display, the contrast limitation experienced with the above-described semi-transmission type of liquid crystal display device is not a problem at all in a bright environment. However, in an environment in which external light, is insufficient, the contrast is insufficient as well. An illumination device may be installed to solve this problem, but unlike the semi-transmission type of liquid crystal display device, an illumination device cannot be installed on the back surface of the liquid crystal panel in the reflection type of liquid crystal display device.
In recent years, there has been a liquid crystal display device in which a touch panel 4 is stacked on the display-surface side of the liquid crystal panel 1 so that information may be externally inputted directly from the display surface of the liquid crystal panel 1. In this kind of touch panel, a spacer is inserted between at least two sheets or substrates which respectively have transparent conductive films formed on their inner surfaces opposed to each other, and a two-dimensional position on the liquid crystal panel is inputted by applying pressure at the position to vary the space between the two sheets or substrates.
The above-described illumination device for a liquid crystal panel has an optimum construction as a device which illuminates a transmission or semi-transmission type of liquid crystal panel from the back surface thereof. However, it is meaningless to apply the illumination device to a liquid crystal display device which uses a reflection type of liquid crystal panel positively utilizing external light in the above-described manner.
In the reflection type of liquid crystal display device, in order that light incident from the obverse-surface side be totally reflected and be again made to exit from the obverse-surface side, a reflector is provided on the back surface or a reflection layer is formed on the inner surface of a lower substrate of the two substrates which constitute the liquid crystal panel.
However, in the reflection type of liquid crystal display device which uses external light as illumination light, a display is difficult to identify in a dark environment in which the external light is insufficient.
If a touch panel for inputting information is stacked on the display surface of the liquid crystal panel, the quantity of transmitted light decreases and the screen of the liquid crystal panel becomes darker and darker.
Incidentally, Japanese Patent Application No. 351794/1997 describes an arrangement in which a touch panel for inputting information and an illumination device for a liquid crystal panel are formed integrally with each other. However, in the arrangement disclosed in Japanese Patent Application No. 351794/1997, since the surface of an illumination-and-touch panel on which a light guide is provided is not flat, the connection between electrodes of the touch panel is far from good. In addition, in the arrangement disclosed in Japanese Patent Application No. 351794/1997, the surface on which the light guide is provided needs to be formed into a staircase-like shape so that the light of a light source is guided to the liquid crystal panel, with the result that the surface on which the light guide is provided cannot be made flat.
One object of the present invention is to improve the coordinate recognition characteristics of an information input device (touch panel) which is provided on a liquid crystal panel.
Another object of the present invention is to provide a reflection or semi-transmission type of liquid crystal display device in which a touch panel is stacked on the display surface of its liquid crystal panel and its entire effective display area is uniformly illuminated by a front illumination system so that a high-quality image display can be obtained and the brightness of the screen can be improved.